FIG. 11 is an explanatory view illustrating the configuration of a key portion at a conventional operating portion used in a portable terminal. A top view of the conventional key portion is illustrated in FIG. 11A, and a sectional view of the conventional key portion is illustrated in FIG. 11B.
The key portion illustrated in FIG. 11 includes a flexible key sheet placed in electrical insulation relative to a substrate 6 and having a recess with respect to the substrate 6, and a metal plate 9 secured in the recess as by an adhesive sheet or the like.
Further, as illustrated in FIG. 11B, the edge of the metal plate 9 is in contact with a pad face 7 formed on the substrate 6, and the central portion of the metal plate 9 is disposed so as to be displaceable in directions toward and away from the substrate 6. When the central portion of the metal plate 9 is pressed toward the substrate 6, the central portion of the metal plate 9 contacts a pad face 8 formed on the substrate 6. FIG. 12 is an explanatory view illustrating the central portion of the metal plate 9 in the pressed state.
When the metal plate 9 conducts electricity owing to contact with the pad face 8, a controller of the portable terminal detects the fact that the key portion has been pressed.
Patent Document 1 describes a switch in which a piezoelectric element is provided on one surface of a substrate. Ultrasonic waves within the substrate that change if a finger touches the other surface of the substrate are detected, whereby contact with the finger is detected.    [Patent Document 1] Japanese Patent Application Laid-Open No. 7-282699 (paragraphs 0018 to 0043; FIG. 1).